Date: Thursday, March 17, 2005
Time: 4:00 pm
Location: NSH 123

Speaker: Dr. David Odde

From: Department of Biomedical Engineering, University of Minnesota

Title: Splitting the genome: Modeling of kinetochore microtubule dynamics in budding yeast

Abstract:
The self-assembly and disassembly dynamics of microtubules (MTs) are central to the proper segregation of chromosomes during mitosis. In particular, a so-called kinetochore microtubule (kMT) physically associates with a chromosome via its plus end to then mediate chromosome movement coupled to the addition and loss of tubulin subunits from the kMT plus end. Given its importance to mitotic chromosome movement, we asked whether tubulin addition and loss from kMTs is regulated in any way. To address this question we developed a Monte Carlo simulation of the kMT dynamics assuming that they obey "dynamic instability", the stochastic biphasic switching from a persistent assembling state to persistent disassembling state ("catastrophe") and back again ("rescue"). With this model we could predict the distribution of kMT plus ends, which was then compared to the observed distribution obtained by fluorescence microscopy of budding yeast that express a GFP fusion of a key kinetochore component (Cse4p). We found that simple dynamic instability failed to account for the observed distribution, regardless of the parameter values. Instead, we found that kMT dynamics are influenced by two phenomena: 1) catastrophe that increases with increasing distance from a pole (i.e. a "catastrophe gradient") and 2) rescue that increases with increasing tension between sister kinetochores. We speculate that the catastrophe gradient originates from a pole-bound kinase antagonized by a nucleoplasmic phosphatase, that both operate on a kMT assembly regulator whose activity is dependent on its phosphorylation state. We further speculate that the tension-dependent rescue effect is at least partially mediated by physical constriction of the kinetochore around the kMT to promote protofilament straightening. Together, these two phenomena enable the characteristic bi-oriented spindle to form, and explain how tension is generated consistently across all sister chromatid pairs, which is an important characteristic of proper spindle assembly used by the spindle checkpoint. The two phenomena also serve to illustrate general mechanisms of MT-mediated intracellular morphogenesis where spatial gradients in MT regulation control the orientation of the MT array.

Reference:
Sprague, B.L., C.G. Pearson, P.S. Maddox, K.S. Bloom, E.D. Salmon, and D.J. Odde, Mechanisms of microtubule-based kinetochore positioning in the yeast metaphase spindle. Biophysical Journal, 2003. 84: p. 3529-3546.